Denske: I haven't had a serious and persistent diatom bloom to try it on, but it's eliminated any small traces, and several others reported the treatment wipes larger quantities out effectively. I also took a peek at the subject lines for your posts within the past two months. Unless I missed something in that brief check, you haven't posted up a thread providing tank details and requesting help for the diatom issue. It's always a good idea to try and diagnose any underlying problem prior to trying a chemical treatment. Or at least at the same time you start the treatment, if the algae has gotten so bad that you need to buy some time.